Why is the camera drifting when following a path and tracking an object?
You have two cameras on the scene.

You have a marker in the timelilne that controls switching the active camera to camera.001 as the active one at a certain frame.

On the second camera you have a different kind of animated motion. 

You can delete the marker or disable the 2nd camera for rendering. You disabled the visibility of it but not the renderability (the camera icon)